Personal cleansing compositions of today are designed to give good cleansing effect in addition to other benefits such as moisturization to the skin. In order to achieve superior cleansing and moisturizing skin benefit, traditionally, very high levels of occlusive agents like petrolatum have been used. For example, Dove Cream Oil (Unilever) and Olay Ribbon body washes (Proctor and Gamble) employed as high as 30% of emollient oils. There have been continuous efforts to provide skin and hair care benefits from cleansing compositions through actives like silicone oil, petrolatum and a variety of triglycerides. Some cleansing compositions have been reported to use as high as 50% of emollients. (Chambers et al. U.S. Pat. No. 5,612,307 and Pavvada et al. U.S. Pat. No. 5,965,500).
Recently Tsaur et al. have revolutionized cleansing technology by inventing various combinations of mild surfactants. U.S. Pat. No. 8,263,538 teaches a combination of N-acyl amino acid surfactants, such as sodium N-cocoyl glycinates and sodium N-cocoyl sarcosinates, with amphoteric/zwitterionic surfactants for ‘super mild’ personal cleansing compositions that showed total irritation score using patch test methodology to be less than 75% relative to 0.5% aqueous sodium dodecyl sulphate solution.
Yet another recent patent by the same group (Tsaur et al. U.S. Pat. No. 8,268,767) reports combinations of cleansing surfactants that not only cleanse and deliver the benefit agents but are ‘super mild’ to skin. The said compositions comprise of both O-acyl isethionates and N-acyl amino acid surfactants for liquid cleansers of both skin and hair. The compositions are said to be ‘super-mild’ to skin when tested by standard patch test method on human volunteers. The above mentioned patent also reveals exceptionally mild compositions that deliver the occlusive moisturizers very effectively. Similarly, U.S. Pat. No. 8,114,824 discloses compositions with O-acyl isethionates and N-acyl amino acid surfactants that could accommodate maximum amount of moisturizer. In summary, the surfactant systems comprising of O-acyl isethionates and N-acyl glycinates or N-acyl sarcosinates as main ingredients have been reported to be very good cleansing systems that are ‘super mild/gentle’ on skin and excellent delivery vehicles of actives (benefit agents like emollients, silicones, triglycerides and petrolatum). The above mentioned patents teach that the ‘super mildness’ and ‘delivery benefits’ are obtained due to the exceptional synergy exhibited by the combination of these two classes of mild surfactants, namely, O-acyl isethionates (Formula I) and N-acyl amino acid surfactants (Formula II).
Out of these mild surfactants, N-acyl aminoacid surfactants, namely, alkali metal or ammonium salts of N-acyl glycinates, sarcosinate, taurates, glutamates, alanates etc are commercially available in solid form as well as in the form of aqueous solution for the formulators since they are highly water soluble. However, contrary to this, the corresponding N-acyl isethionates exhibit virtually no solubility in water. For example, sodium cocoyl isethionate has 0.1% solubility in water at 25° C. (Assessment Plan for Fatty acids, coco, sulfoethyl esters, sodium salts (sodium cocoyl isethionate prepared by Keller and Heckman LLP, November 2006). In addition, all alkali metal O-acyl isethionates have very high melting points of over 200° C. Incorporation of extremely high melting alkali metal O-acyl isethionates in personal care formulations (body wash, face wash) necessarily involves heating of the formulations which is sometimes not advisable due to thermo-susceptibility of other sensitive ingredients in addition to expending the energy for heating the entire mass. To overcome this difficulty of incorporation of alkali metal O-acyl isethionates in their solid form, researchers have created formulator-friendly ‘Liquid Delivery Systems’ (LDS) wherein alkali metal salts of O-acyl isethionates are solubilzed in aqueous medium with the help of other surfactants and hence remain fluid and easy to incorporate (cold mixing) in the personal care formulation. EP0964674 A2 and U.S. Pat. No. 5,925,603 (Rhodia, Paul D'Angelo) report concentrated ‘Liquid Delivery Systems’ where in O-acyl isethionates are dispersed with alkyl imidazoline amphoteric surfactants and with anionic surfactants. US Patent application 20090062406 and U.S. Pat. No. 5,415,810 (Clariant, Matthias Loeffler) teach aqueous concentrated blends wherein O-acyl isethionates are made deliverable with N-acyl taurate and betaines. U.S. Pat. No. 7,879,780 reports stable cleansing compositions with O-acyl isethionates with betaines and sulphosuccinates. A recent patent application by Galaxy Surfactants reports liquid delivery systems for O-acyl isethionates using N-acyl glycinates and alkyl betaines (U.S. patent application Ser. No. 13/749,458, A Jawale et al.). The examples of commercially available ‘liquid delivery systems’ for sodium cocoyl isethionate are 1) Chemoryl™SFB, Lubrizol (sodium cocoyl isethionate, laureth sulphosuccinate and cocomidopropyl betaine), 2) Hostapon SCB, Clariant (sodium cocoyl isethionate, cocobetaine and 3) Miracare Plaisant, Rhodia (sodium cocoyl isethionate, cocoamphoacetate, and sodium cocoyl taurate).
All liquid delivery systems (LDS) that have been reported so far basically use other anionic or amphoteric/zwitterionic surfactants (‘Solubilization sodium cocoyl isethionate’; J. Z Sun, M. C. E. Erickson, and J. W. Paar, J. Cosmetic Science 54, 559-568, 2003) to covert solid alkali metal O-acyl isethionates into either soluble or dispersible in aqueous medium. The biggest disadvantage of these ‘liquid delivery systems’ are that they are based on SOLID form of salts of O-acyl isethionates that are available in the form of needles, pastille, powder or granules. Making solid form of O-acyl isethionates itself involves a reaction at 225° C. and handling of molten mass to convert that into solid of desired physical form. These liquid delivery systems of O-acyl isethionate are created to ease the job of personal care formulators. However, the manufacturers of liquid delivery systems have to deal with dissolution of hard, virtually water-insoluble, high-melting (above 200° C.) O-acyl isethionates. This is achieved by heating and mixing of solid O-acyl isethionates with other surfactants, either anionic or amphoteric or both. Industrial process for manufacturing O-acyl isethionate involves very high temperature of 200 to 240° C. wherein fatty acid is reacted with alkali metal salts of hydroxyl ethyl sulphonate (Friedman, M. 2004. Chapter 5: Chemistry, Formulation, and Performance of Syndet and Combo Bars. In, Spitz, L. (ed), SODEOPEC Soaps, Detergents, Oleo-chemicals, and Personal Care Products, AOCS Press, Champaign, II). The typical industrial procedure involves use of at least 1.4 to 2.0 molar equivalence of fatty acids with respect to alkali metal isethionate (40 to 100% excess of fatty acid) to effect the acid catalyzed esterification. The excess fatty acid is recovered after the desired degree of esterification. This process is known as DEFI (directly esterified fatty acyl isethionate) in literature. The esterification between hydroxyl ethyl sulphonate and fatty acid is effected at temperatures above 200 to 225° C. and excess fatty acid is typically recovered at a temperature of more than 200° C. under high vacuum. The recovery of fatty acid is never completely done and hence commercially available alkanoyl isethionates (O-acyl isethionates) are about 65 to 85% purity. Some amount of fatty acid (10 to 15% w/w of the final composition) needs to be left in the reaction mass so that the reaction mass with 65 to 85% O-acyl isethionate remains fluid at 200 to 240° C. This facilitates the transfer and conversion of molten mass into needles or prills by complex technology to ensure there is no deterioration of bulk of the molten mass at very high temperature. With every form of solid form (granule, powder or prill) of O-acyl isethionate that is produced there is a certain degree of dusting associated and that warrants special precaution as inhalation of this dust is a serious health hazard. Since the O-acyl isethionates are very high melting compound and virtually no solubility in water, these anionic surfactants are difficult to incorporate in personal care formulations. Thus, the Liquid Delivery Systems use commercially available solid form of O-acyl isethionates and convert solid O-acyl isethionates in liquid/fluid deliverable form using other surfactants. Another major disadvantage of the liquid carrier systems for O-acyl isethionates is that commercially available and patented LDS come with significant amount of certain surfactants. This restricts the usage of so called ‘liquid delivery systems’ since a formulator may not want to have the other surfactants that are used to create liquid deliverable O-acyl isethionate systems. The marketed ‘liquid delivery systems come with imidazoline amphoterics or alkyl betaines or even anionics like fatty alcohol ether sulphates and the amount of these surfactants that are employed are too high relative to the amount of O-acyl isethionates that is solubilized or dispersed in water. In addition, surfactants like fatty alkyl betaines (used by Galaxy Surfactants and Clariant) are very expensive since they are made from an expensive raw material in the form of tertiary fatty amines with one long chain and two short alkyl chains on the tertiary nitrogen. Finally, liquid carrier systems comprising only combinations of ‘super-mild’ O-acyl isethionates and N-acyl amino acid surfactants have not been reported so far. The obvious reason is that by using solid form of O-acyl isethionates it not possible to create fluid and stable systems. For example, it is not possible to create aqueous homogeneous solution of sodium cocoyl isethionate and sodium cocoyl glycinate in 1:1 molar ratio from commercially available solid form of sodium cocoyl isethionate.
In view of O-acyl isethionates and N-acyl amino acid surfactants being the core of ‘super mild’ personal cleansing systems there is a need in the art 1) to make compositions using only these two ‘super-mild’ surfactants that are easy to incorporate in personal care formulations and 2) with an elegant process of manufacture that would have the flexibility of changing the ratio of O-acyl isethionates to N-acyl amino acid surfactants.